The present invention relates to a transmission system for implanted hearing aids of the type having at least one implanted excitation electrode, an acoustic convertor with a corresponding circuit, a high frequency transmitter cooperating with the convertor and transmitting by means of a transmitter coil, an implanted receiver unit including at least one receiver coil and being electrically connected to the excitation electrode.
From prior art it is known how to excite electrically the inner ear of totally deaf patients whose auditory nerve is still intact. For this purpose, a mono- or multi-electrode is inserted in the cochlea of the patient, the electrode being provided with distributed contact members which are excitable via a receiver unit by high frequency signals generated by an external transmitter unit, thus producing an acoustic sensation in the auditory nerve of the patient. The acoustic convertor converts the received sound waves into electrical signals which are processed in a suitable electrical evaluation circuit whose construction is well known in the art and therefore it will not be discussed in connection with this invention. The evaluation circuit shapes the electric acoustical signals, decodes these signals and distributes them into a series of frequency channels selected such that the corresponding signals transferred by the implanted high frequency receiver excite the auditory nerve of the patient in the most natural way that is to simulate the natural sound conversion in the inner ear in the most accurate way.
When transmitting signals generated by an external high frequency transmitter and coupling these signals via a transmitter coil to a coil of the implanted high frequency receiver, there is a problem of an adjustment of the inductive coupling between the transmitter and receiver coils. It is desired to provide a coupling which minimizes the consumption of energy from the minute battery pertaining to the transmitter and receiver units. In prior art solutions in order to place the transmitter coil in close proximity to the receiver coil, the transmitter coil has been arranged on a crown yoke, on a head band, in a frame for glasses or in a separate ear yoke in such a manner as to contact the skin behind the auricle of the patient opposite the implanted receiver coil. The disadvantage of this prior art arrangement is the fact that an exact alignment of the transmitter coil with the receiver coil is obtained only accidentally and due to unavoidable shaking resulting from movements of the patient the aligned position in practice cannot be maintained for a prolongated period of time. Attempts have been made to create a centrally aligned coupling position of the transmitter and receiver coils by means of a magnetic holder for the receiver coil whereby an extremely strong permanent magnet made of lanthanide series substances is provided in the center of the implanted receiver coil and a magnetizable counter piece is arranged in the center of the external transmitter coil so that the latter is automatically centered and held in position by the strong magnetic forces. In this solution the transmitter coil contacts the skin of the patient under pressure thus rendering it susceptible to inflammation in the range of the implanted receiver coil where the patient had undergone a surgical operation.
In addition conventional arrangements of transmitter and receiver coils have cosmetic and psychological disadvantages because the transmitter and its holder is conspicuous and adds to the psychological discomfort of deaf persons during social communication.